Family Ties

The sun is shining. You’re consuming spiked seltzer in great quantities (jk that’s just me). Summer is a time for family and for love. This week, we just want to remind you not to mix the two.

98 Pedigrees

Our tale begins in my ninth grade history classroom. One day, while teaching us about the Habsburgs (a German-Austrian royal family that ruled great swaths of Europe between the beginning of the twelfth century and 1700), my teacher pulled up a horrifying drawing of a person who looked more animal than human. This, he said, was what the Habsburg offspring looked like toward the end of their reign due to a little thing called inbreeding. The picture may have been a cartoonish overdramatization, but the science behind it was real. 

A Habsburg who suffered unfortunately from a receding hairline is lucky the genes she inherited weren’t more debilitating. From The Runaway Bunny.

Marriages between relatives have been common in royal families throughout history, but the Habsburgs took inbreeding to a whole new level. Never before had relatives so close to one another married with such frequency. Intra-familial marriages strengthened their empire for a time, but eventually led to its demise.

By the end of the Habsburg reign in 1700, their empire included the majority of Europe. From The Map Archive.

Too Close for Comfort

Not only is inbreeding morally repugnant, it’s genetically disadvantageous. Think of purebred dogs, inbred to maintain purity, that end up with a multitude of genetic defects. The same thing happened to the Habsburgs. At conception, you inherit a set of genes from each parent in the form of a set of chromosomes. These chromosomes are further able to swap pieces of genes with one another in order to increase genetic diversity. Aside from making you totally unique, this serves the important evolutionary advantage of minimizing the effect of any deleterious genes you may have inherited. For example, if your dad gave you a defective version of a gene (cough, my lactase enzyme), your mom’s copy can pick up the slack. But when closely related individuals have children, there is more of a chance that both parents have the same genetic defect. As a result, inbreeding increases the risk of rare genetic mutations.

A family tree of some of the Habsburgs. Notice some big names like Marie Antoinette and Marie Medici. Also notice that the arrows are overlapping A LOT. From Nature.

There’s actually a mathematical way to measure this, called the inbreeding coefficient. This coefficient, with possible values ranging from 0 to 1, represents the probability that two copies of a gene inherited from each parent are identical due to a common ancestor. A study published by researchers from the University of Santiago de Compostela in Spain in 2009 found that the inbreeding coefficient for the last Habsburg ruler, King Charles II of Spain, was 0.254, higher than it would be if he was the result of a mating between parent and child.

This graph shows the increase in inbreeding coefficients across Hapsburg rulers from 1482 to 1661. From Alvarez et al.

El Hechizado

Although the Habsburgs benefited politically from their strategic marriages, their biology began to weaken. Royal portraits regularly show members of the family with a pronounced chin dubbed the “Habsburg jaw.” Another group of researchers from Spain were able to find a correlation between a family member’s inbreeding coefficient and the degree of their jaw deformity, thought to be a genetic trait called “mandibular prognathism”, based on paintings and historical records. Inbreeding also increased the risk of child mortality for Habsburg offspring, with the probability for a child to survive to 10 years old decreasing to less than 50% as the inbreeding coefficient reached 0.254.

You can appreciate from this figure that as the degree of inbreeding increased, a child’s probability of surviving to 10 years of age decreased. From Alvarez et al.
Charles II’s Habsburg jaw in all of its glory. From The Mirror.

Ultimately, the effect of inbreeding on fertility brought down the Habsburgs’ centuries-long rule over Europe. The subtle facial features that were seen throughout the family dynasty became debilitating deformities for Charles II, the last Habsburg king. He was severely mentally and physically disabled, barely able to walk or talk until he was 10, and had lifelong difficulty speaking and eating due to his jaw deformity. He was known as “El Hechizado,” or “The Hexed,” due to his impairments, likely caused by the inbred genes he inherited. Although he was married twice, he had no children, and his first wife made sure to tell everyone about his fertility issues (another symptom that could be explained by his highly inbred genetic profile). Thus, when Charles II died in 1700, the family empire died with him. You can conquer land, you can win wars, you can dominate trade. But at the end of the day, you can’t fight nature.


ABRI, U. (n.d.). Retrieved from

Alvarez, G., Ceballos, F. C., & Quinteiro, C. (2009). The Role of Inbreeding in the Extinction of a European Royal Dynasty. PLoS ONE,4(4). doi:10.1371/journal.pone.0005174

Royde-Smith, J. G. (2019, October 16). House of Habsburg. Retrieved from

Thulin, L. (2019, December 04). The Distinctive ‘Habsburg Jaw’ Was Likely the Result of the Royal Family’s Inbreeding. Retrieved from

How Inbreeding Doomed the House of Habsburg (2020, April 08). Retrieved from, R., Ceballos, F. C., Al-Soufi, L., González-García, R., Moreno, C., Moreno, M., . . . Álvarez, G. (2019). Is the “Habsburg jaw” related to inbreeding? Annals of Human Biology,46(7-8), 553-561. doi:10.1080/03014460.2019.1687752

Vilas, R., Ceballos, F. C., Al-Soufi, L., González-García, R., Moreno, C., Moreno, M., . . . Álvarez, G. (2019). Is the “Habsburg jaw” related to inbreeding? Annals of Human Biology,46(7-8), 553-561. doi:10.1080/03014460.2019.1687752

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